The present invention relates to a method of forming bumps as electrical, mechanical and thermal connections or contacts on a semiconductor device.
There are many examples as to the method of forming bumps on a semiconductor device. For example, metal is deposited by plating to form bumps on the pads of a semiconductor device (plating method); a solder paste is printed on the pads of a semiconductor device, heated so that solder particles within the paste are melted, and thereafter solidified to form bumps on the pads (printing method); and one end of gold wire is bonded to a pad, and then the wire is cut so that the bonded part of wire is left as a bump (wire bonding method). In addition, as disclosed in U.S. Pat. No. 5,284,287, solder balls are sucked in the cavities of a pick-up tool by vacuum, placed on the pads of a semiconductor device and heated to melt, and solidified to form bumps (conventional type solder ball method).
These conventional methods, however, have the following drawbacks. In general, the larger the cubic volume of the bumps, the longer the life of the connection between the semiconductor device and an electronic circuit board through the bumps can be extended. In the plating method and printing method, however, it is difficult in principle to form bumps of enough cubic volume. Moreover, since the heights of the bumps scatter, all the bumps cannot be properly connected between the semiconductor device and, the electronic circuit board. In the wire bonding method, the material of the wire is limited to only a particular one such as gold. Also, since bumps are produced one by one, it takes a very long time to produce many pads as for a semiconductor device having tens of thousands of pads.
In the conventional type solder ball method, the scattering of the bump height is small, and bumps of enough volume can be produced, but the pick-up tool for use in sucking balls by vacuum to hold is complex in structure and requires a delicate perforating technique for very fine holes or cavities when it is produced. Since this tool becomes expensive in proportion to the number of holes required, the cost of forming bumps increases when the tool is used for a semiconductor device that needs a large number of bumps.